Herein, the term "electronic module" means an IC or other form of packaged electronic circuitry having a limited number of input/output pins.
It is often required that an electronic module be able to enter a test mode, in which one or more functional characteristics of the module are tested and/or modified. This can be done by including a separate pin which receives a signal for initiating the test mode. But where a pin cannot be dedicated solely to the testing function (particularly in IC's) the test mode has been conventionally initiated by applying to a pin, which pin is also used for a function other than testing, a voltage which is outside its normal operating range (typically above V.sub.cc or below ground). That voltage is detected in the module and used to trigger the test mode.
Although the last-mentioned technique can be used in certain applications, some IC's have such a small number of input/output pins, and/or operate in such a critical environment, that this conventional technique cannot always be used reliably. For example, an automotive voltage regulator that is constructed as an IC may have only three input/output pins that are available for normal operation of the device and for use in initiating the test mode. Moreover, the environment in which the voltage regulator is used in sometimes too noisy to reliably use an abnormally high or low voltage on one of the pins to initiate the test mode.